Effect Of Superheated Steam Treatment On Qualities Of Wheat And Wheat Flour

In the process of planting,harvesting,storage,processing and transportation,wheat grains are often contaminated with microorganisms,thus affecting the storage period and leading to the decline in qualities of the flour and end products.Therefore,the microbial decontamination of wheat grains is needed.In recent years,as the material of whole grain products,whole wheat flour has gained more and more attention and was popular among consumers.However,both the microorganism level and enzyme activity in whole wheat flour are high,and the sterilization and enzymes inactivation are needed to improve the quality and prolong the shelf life of whole wheat flour.In addition,heat treatment is a simple and practical flour modification method.Superheated steam(SS)technology is a clean and efficient heating method,which has been widely used in food processing and other fields in recent years.The use of SS on food processing has obvious advantages such as evenly heating,fast temperature increasing and nutrients loss preventing as well as good quality of the end products.Meanwhile,it also has benefits of safety,environmental protection and energy saving.So this thesis selects the SS as heating medium.Depending on different processing demands,the wheat and its components(flour,wheat bran)were treated respectively,and thus,killing microorganisms,inhibiting enzyme activities,and improving storage characteristic and functional properties.In addition,changes of the structure,physicochemical properties and nutritional qualities of wheat were also systematically studied.The aim of this study is to provide some theoretical guidance for the application of SS in food processing,and additionally,to provide a new method for grain sterilization,enzymes inactivation and flour functional properties improvement.The main results of this paper are as following:(1)SS treatment on wheat could effectively kill the microorganisms,including bacteria,moulds and Bacillus spp..SS treatment could kill bacteria,moulds and Bacillus spp.in wheat grain quickly.The steam flow rate at 15.0 m3/h was more efficient than at 7.5 m3/h during sterilization treatment.The residual levels of bacteria decreased rapidly during the first 10-40 s especially at high SS temperatures,and then followed by a 'tailing' period with increasing of treatment time.The inactivation effect of SS processing at 140 ? with shortened steam condensing period was comparable to that at 110 ? which has a lower enthalpy.Higher temperatures(170 and 200 ?)were more effective on elimination of Bacillus spp.than the lower ones(110 and 140 ?).After a SS treatment at 200 ?,with 15.0 m3/h for 80 s,approximate 99.9%of total bacteria and 81.8%of Bacillus spp.were reduced,respectively.Moulds were completely removed by SS processing at 110 ? or higher temperatures,for 30 s or longer.Moisture tempering could increase the levels of microorganisms in grain,while promoted the microbial decontamination activity.SS should be applied to clean wheat between the processes of milling and tempering.(2)SS treatment on wheat could improve the flour and made noodle qualities.Processed wheat grains with SS could increase the particle size of the milled flours,decrease the amylase activity,make the starch partial gelatinized and lead to protein-protein and protein-starchcrosslink reacted;110-170 ? of SS treatment on wheat grains for 1-3 min could decrease starch damage contents of the milled flours,increase viscosities,make flours brighter and dough stronger,thus improving the qualities of flour.However,long times(2-3 min)of SS treatment at 200 ? could increase starch damage contents,decrease flour viscosities and brightness values,destroy the gluten structure and make it hard to form dough,thus damaging the qualities of milled flours in a certain degree.Noodles made from wheat modified by SS treatment under optimal conditions exhibited higher firmness,springiness,and chewiness,while lower cooking loss compared with those made from untreated wheat.(3)SS treatment could improve nutritional properties and storage characteristics of wheat bran.SS treatment(7 min)was more efficient than conventional hot air(HA)treatment(16 min)at wheat bran(WB)enzymes inactivation processing;Both stabilization treatments could maintain the proximate composition of WB.However,SS processing with oxygen-free environment avoided the oxidation of fat and fatty acids in WB;Compared with hot air-stabilized wheat bran(HA-WB),superheated steam-stabilized wheat bran(SS-WB)exhibited a brighter color,higher extractable phenolic compounds contents and stronger antioxidant activities as well as lower peroxide value and higher unsaturated fatty acid contents.The viscosities and dough rheological properties of the whole wheat flour reconstituted from SS-WB were poorer than that reconstituted from native WB,but better than whole wheat flour reconstituted from HA-WB;SS stabilization treatment significantly extended the shelf life of WB.The quality of SS-WB stored at room temperature was better than that of native WB even stored at low temperature(4 ?).SS-WB could be a potential alternative to native WB and HA-WB for recombinant whole wheat flour.(4)SS treatment could improve structure and physicochemical properties while decrease starch in vitro digestibility of wheat flour.Processed wheat flours with SS could induce interactions between starch-protein and starch-lipid,make amylose and amylopectin cross-linked,and reduce the amylose leaching.These changes made the structure of SS-modified flours as well as starch granules more stable,and prolonged water absorption time of starch granules,as well as enhanced water holding ability and viscosities of wheat flour.While for the moistened flours,SS treatment also increased thermal transition temperatures as well as slowly digestible starch(SDS)and resistant starch(RS)contents.Elevated moisture level promoted the modification of both structure and physicochemical properties of flour during SS treatment.(5)SS treatment could alter the protein structure and improve dough rheological properties of wheat flour.SS treatment on wheat flours resulted in protein aggregation accompanied by decreasing in free sulfhydryl contents and increasing in unextractable polymeric proteins(UPP)contents as well as alteration of secondary structures(transformed from ?-turn and random coil to ?-helix and ?-sheet).These changes led to the significant increasing in strength as well as elastic modulus and viscous modulus of the developed dough.Additionally,alteration of UPP contents also showed positive correlations with dough rheological parameters.Elevated moisture level in flours promoted the modification of protein structures during SS treatment and thus leading to much higher enhanced dough rheological parameters.